HR: 0800h
AN: U21A-0703 [Abstracts]
TI: $^{40}$Ar/$^{39}$Ar Age Calibration of the Lacustrine Sediments at Kariandusi, Central Kenya
Rift
AU: * Deino, A L
EM: al@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709
United States
AU: Trauth, M H
EM: trauth@geo.uni-potsdam.de
AF: Institut fr Geowissenschaften, Universitt Potsdam, POB 601553, Pottsdam, D-14415
Germany
AU: Bergner, A G
AF: Institut fr Geowissenschaften, Universitt Potsdam, POB 601553, Pottsdam, D-14415
Germany
AU: Potts, R
EM: Potts.Rick@nmnh.si.edu
AF: Smithsonian Inst., Dept. of Anthropology, MRC 122
National Museum of Natural History, Washington, D.C 20560
United States
AB:
The Kariandusi site in the Nakuru-Elmenteita Basin, central Kenya Rift, contains a 31 m thick sequence of well-stratified
diatomites and tuffs. It is significant in the overall late Cenozoic climate picture of East Africa in that Kariandusi is one
of three sites (along with Olorgesailie, southern Kenya, and Afar, Ethiopia) that record a major wet period at $\sim$1000
ka. The diatomite assemblage records the complete history of a large, deep lake, from lowermost units containing
benthic-epiphytic flora, followed by deep-lake genera, and succeeded in the higher parts of the section by an assemblage that
indicates lake regression with higher alkalinity. The diatomite rests directly on the widespread Gilgil trachyte lava
flows, dated at Kariandusi by laser $^{40}$Ar/$^{39}$Ar total-fusion of anorthoclase phenocrysts at 0.985 $\pm$ 0.003 Ma
(1$\sigma$).The main diatomite is conformably overlain by a 0.7 m thick tephra horizon that apparently represents a direct
airfall into the lake. The age of this unit by single-crystal $^{40}$Ar/$^{39}$Ar dating of anorthoclase is 0.973 $\pm$ 0.003
Ma. It is succeeded by an intercalated sequence of diatomites and tephra about 4 m thick.
Previous $^{40}$Ar/$^{39}$Ar dating (Deino and Potts, 1990, JGR 95:B6: 8453-8470) indicates that the Kariandusi diatomite
sequence was deposited at approximately the same time as lacustrine sediments in the lower part of the Olorgesailie Formation
(bracketed between 1000-980 ka), 135 km to the south within the Kenya Rift. Limited tephra correlation based on
trace-element analysis of glass separates suggests that the Olorgesailie lacustrine sequence may be equivalent to the upper
part of the Kariandusi diatomite sequence, or slightly younger. Further work, including dating, tephra correlation, and
diatomite assemblage studies, may eventually permit precessional-level assignment of the lacustrine phases at the two sites.
DE: 9305 Africa
DE: 9604 Cenozoic
DE: 3344 Paleoclimatology
DE: 1035 Geochronology
SC: Union [U]
MN: 2004 AGU Fall Meeting